1. Field of the Invention
The present invention belongs to a technical field of information recording apparatus that records information on an information recording medium such as a CD-R (Compact disc-Recordable), a DVD-R (DVD-Recordable), or a DVD-RW (DVD-Rewritable) and information recording/reproducing apparatus that reproduces information from such an information recording medium. Particularly, the present invention relates to a tracking servo technique and a control of movement of a light beam at the time of random access, etc.
2. Description of Related Art
As an information recording medium on which additional writing or rewriting information is possible, there are known an optical disc such as a CD-R, a DVD-R, and a DVD-RW. On each of those information recording media, tracks for recording information (called “information recording track” or “groove”) and lands for guiding the light beam (called “guide track” or “land”) are spirally alternately formed in the manner adjoining with each other. On the track for recording information, contents information such as video information and/or audio information can be optically recorded. Also, the guide track is provided to cause the light beam to correctly follow the information recording track, especially in a non-recorded area of the disc.
When recording information on the information recording medium or reproducing information from the information recording medium, it is needed to accurately position the light beam on the information recording track. Therefore, in the information recording/reproducing apparatus, the tracking servo control is performed.
In a differential push-pull method known as a representative tracking servo technique, one main beam is irradiated onto the information recording track and two sub-beams are irradiated onto the adjoining guide tracks on both sides of the information recording track on which the main beam is irradiated. The return lights of the light beams irradiated onto the respective tracks are received by a two-divided or four-divided type photo-detector. Push-pull signals corresponding to the three beams are produced from the electric signals output from the photo-detector. After adjusting the signal amplitude based on the difference in quantity of light beams between the main beam and each of the sub-beams, the sum of the push-pull signals corresponding to the sub-beams is subtracted from the push-pull signal corresponding to the main beam, thereby a differential tracking error signal is obtained. The servo control circuit controls the position of the light beam from the pickup so that this differential tracking error signal becomes zero, thereby causing the light beam to trace the information recording track. In this way, the light beam for recording or reproducing information is so controlled as to be exactly positioned on the information recording track.
In such an information recording/reproducing apparatus, so-called “random access processing” (recording/reproduction) can be performed. The “random access processing” is a processing for changing the information recording or reproducing position on the information recording medium. In other words, it means that, during recording or reproduction with respect to an information recording track of the information recording medium, according to the user's command or else, the light beam is moved to another information recording track to perform recording or reproduction with respect to the information recording track after the movement. In this case, as the processing of the information recording/reproducing apparatus, the above-described tracking servo control is once made ineffective, the optical pickup up is moved to the vicinity of a target information recording track on the information recording medium, the tracking servo control is restarted to position the light beam on the target information recording track, and then the recording or reproduction is restarted.
When moving the optical pickup to the vicinity of the target information recording track, generally the number of the tracks that the light beam traversed in the radial direction of the information medium is counted by referring to the tracking signal. The radial contrast signal is produced according to the amplitude or else of an RF signal obtained by moving the light beam in the radial direction of the information recording medium, and it basically indicates the fluctuation in the amplitude of the RF signal depending upon the cross-sectional shape or configuration in the radial direction of the information recording medium. The light beam is moved to the vicinity of the target track by counting the tracking signals and, when closing the tracking servo loop, the position of the light beam with respect to the target track is detected by using the tracking signal and radial contrast signal. Thereby, the light beam is appropriately positioned on the target track and tracking servo loop is smoothly closed.
At this time, if information has already been recorded on the information recording medium, the RF signal is obtained by moving the light beam in the radial direction of the information recording medium, and hence the radial contrast signal is clearly obtained. However, since no clear RF signal is obtained in the non-recorded area of the information recording medium, there is the problem that it is difficult to obtain a radial contrast signal whose amplitude distinctly changes.
For this reason, the inventors of the present invention previously proposed a method of obtaining clear radial contrast signal even in a non-recorded area. Specifically, in the information recording/reproducing apparatus using a 3-beam type optical pickup, two sub-beams are irradiated onto the positions on the information recording medium shifted from a center position of the guide track to obtain, even in a non-recorded area, clear radial contrast signals indicating the configuration of the information recording tracks and the guide tracks. This method is disclosed in Japanese Patent Application No. 11-290225 (Laid-Open under No. 2001-110071) and Japanese Patent Application Laid-Open under No. 2001-110071.
In order to execute the above-described random access processing, it is necessary to obtain a radial contrast signal with high accuracy. On the other hand, in order to perform ordinary recording or reproduction, it is necessary to obtain the above-described tracking error signal (i.e., the differential push-pull signal) with high accuracy. Therefore, it is required to meet both of those requirements in the information recording/reproducing apparatus.
However, as stated above, when adopting the method of irradiating the sub-beams at the position shifted from the center of the guide track, an offset or the like is generated correspondingly to the amount of the shift, with the result that the accuracy of the tracking error signal decreases.
In order to obtain a highly accurate tracking error signal by using the differential push-pull method, it is most appropriate that the two sub-beams have the phases shifted by 180 degrees from each other, i.e., when the main beam is located at the center of the information recording track, the two sub-beams are located on the center of the guide tracks adjacent, on both sides, to the recording track on which the main beam is located. However, if this condition is met, it is impossible to obtain a satisfactory radial contrast signal in the non-recorded area as stated above.
On the other hand, if the two sub-beams are irradiated in the manner being shifted from the center of the guide tracks as in the case of the above-mentioned patent applications, although a preferable radial contrast signal is obtained even in the non-recorded area of information, it inconveniently results that the level of the tracking error signal obtained by the differential push-pull method becomes insufficient, or the accuracy of the tracking error signal is lowered due to the occurrence of an offset, etc.
However, providing separate optical systems in order to obtain a preferable tracking error signal and a preferable radial contrast signal leads to the increase of cost, the increase in size, and the complicated structure, of the apparatus, and hence is not practical. Accordingly, it is preferable to satisfy the both requirements by making use of a single optical system and signal processing system in an information recording/reproducing apparatus.